A rotorcraft is provided with at least one engine that drives a main rotor serving to provide the aircraft with lift and also with propulsion. The rotor comprises a hub that is rotated by a rotor shaft driven by the engine, and a plurality of blades are arranged on the hub.
During their rotary motion, the blades are subjected to a torser of forces and consequently they are subjected to a centrifugal force and also to multiple effects due to flapping, drag, and twisting, where twisting is caused in particular by changes of pitch seeking to modify the angle of inclination of the blades relative to the plane of the hub.
Furthermore, the hub generally has a plurality of arms that are possibly flexible in flapping and that have the blades arranged at the ends thereof. The forces due in particular to centrifugal force are then transmitted by the blade to the non-flexible central zone of the hub.
A first type of method consists in fabricating a blade body and then in fastening the body to the rotor hub via specific fastener attachments.
Documents FR 2 874 852, FR 2 321 997, EP 0 011 330, and FR 2 518 979 describe a plurality of techniques enabling a blade body to be fabricated by winding or braiding composite materials.
It is then appropriate to secure the blade body to a fastener attachment.
According to document FR 2 030 036, for example, winding is used to make a cylindrical element that is to constitute the outer covering of the blade. The cylindrical element is then cut longitudinally into two subassemblies, each subassembly then being placed in a mold.
The spar of the blade is also made by winding. The spar comprises a first block of fiber layers presenting an angle of 45° relative to the longitudinal axis of the blade, and then a second block of fiber layers presenting an angle of 30° relative to said longitudinal axis, and finally a third block of fiber layers presenting an angle of 15° relative to the longitudinal axis.
The spar opens out to the outside at the root of the blade via an opening, and a wound attachment is wedged in said opening and then secured to the rotor hub. Thereafter, the blade is terminated by inserting the spar between the two molds in which the two subassemblies for the outer coating of the blade are placed.
The blade then transmits the forces to which it is subjected to the hub, successively via its spar and its fastener attachment.
That method of fabrication is effective, but nevertheless limited.
Fabrication time is lengthy since numerous steps are implemented, thereby clearly leading to excessive costs.
It is also possible to observe defects at the blade root, since the fastener attachment is liable to move relative to the spar.
A second type of method consists in securing the fastener attachment to the blade spar during fabrication of the spar, and not subsequently.
Document U.S. Pat. No. 3,923,422 describes such a technique.
At the blade root, the spar is wound around a fastener attachment that has a vertical axis substantially parallel to the axis of rotation of the rotor.
Nevertheless, in order to make the vertical winding, the spar needs to be twisted. Such twisting through about ninety degrees presents the drawbacks of possibly being destructive insofar as a spar for a composite material blade is constituted mainly by unidirectional fiber tapes.
Furthermore it is difficult for the unidirectional tapes that come from the twisted section of the distributed spar to be placed flat against the suction and pressure sides of the blade.
Twisting can then lead to poor reproducibility from one blade to another both in terms of shape and also of mechanical and vibrational characteristics.
Furthermore, if it is necessary to arrange filler elements in the blade, they must be of shape that is complex and they will vary from one blade to another since the twisting of the spar is not reproduced identically. It then becomes difficult to automate blade fabrication.
In addition, the problem of anchoring the fastener attachment remains.
It is then possible to envisage winding the spar “horizontally”, in accordance with the proposal described in document EP 0 657 646.
According to that document, the blade is provided with a metal fastener attachment at its blade root and with a metal fitting at its free end.
The fastener attachment and the metal fitting are placed in clamping heads of a winder machine. Unidirectional fibers are then wound horizontally around the fastener attachment and the metal fitting. Those fibers are thus substantially parallel to the longitudinal axis of the blade and they are capable of taking up the centrifugal forces that are exerted on the blade.
The winder machine then lays sheets of crossed-fibers between the fastener attachment and the metal fitting.
Front and rear streamlined elements are then put into place and covered in a sheet of crossed-fibers.
Consequently, it remains possible for the fastener attachment to move along a transverse axis that is perpendicular to the longitudinal axis of the blade, and that can lead to the blade being rejected.